Method and device for determining a bending angle of a rotor blade of a wind turbine system

ABSTRACT

A method for determining a bending angle of a rotor blade of a wind turbine system includes step of reading in an acceleration signal which represents an acceleration of the rotor blade acting essentially perpendicularly with respect to a rotor plane. In addition, the method includes a step of determining the bending angle of the rotor blade of the wind turbine using the acceleration signal.

The present invention relates to a method and a device for determining abending angle of a rotor blade of a wind turbine system in accordancewith the independent patent claims.

Wind energy systems are controlled via the adjustment of the rotorblades about their longitudinal axis, and by the generator torque.Controlled variable for the pitch control is the rotor rotational speed,and the manipulated variables are the pitch angles of the rotor blades.The collective pitch control CPC is used with conventional systems.Here, the three rotor blades are all adjusted with the same pitch angle.In the case of wind energy systems with a horizontal axis and at leasttwo rotor blades, synchronous adjustment of the blade angles is used tocontrol the rotational speed above the rated wind speed so that bychanging the incidence angle the aerodynamic lift, and thus the drivetorque, are reduced in such a way that the system can be operated in theregion of the rated rotational speed. Given wind speeds above theswitchoff speed, this blade adjustment mechanism is additionally used asa brake by setting the blades in a fashion nose into the wind so thatthe rotor can no longer supply any appreciable drive torques. Because ofasymmetric aerodynamic loads, pitch and yaw torques on the nacelle areproduced in the case of this collective blade adjustment. The asymmetricloads result, for example, from wind shears in a vertical direction(boundary layers), yaw angle errors, gusts and instances of turbulence,damming of the flow at the tower, etc. A known approach to reducingthese asymmetric aerodynamic loads consists in adjusting the incidenceangle of the blades individually (Individual Pitch Control=IPC). Thiscontrol approach requires determination of the bending torques (inparticular, flapping bending torques) that prevail at the rotor bladeroot. The bending torques then serve as controlled variable for theindividual blade adjustment. Strain gauge sensors that are applied atthe rotor blade root can be used to determine the bending torques. Theproblems in the case of the strain gauge sensors consist in theapplication and risk of breakage, and in the short service life.

Other methods such as are disclosed, for example, in WO 2008/041066 orDE 197 39 164 B4 determine the pitching and yawing torques by measuringthe nacelle acceleration via gyrometers or by means of sensors that usedistance measurements to measure the deformations, occurring during theloads, of system parts, and thereby determine the loads. The bladebending torques are very well suited as controlled variable from thepoint of view of the IPC. However, it has not so far been possible tofind any measurement technique suitable for continuous use. Fiber Braggsensors laminated into the blades to measure torques cannot be exchangedin the case of a defect, while strain gauge sensors bonded on have a fartoo short service life. Both methods additionally have the problem thatthe measurement is performed only locally on the blade. Localinhomogeneities in the laminate therefore lead to measuring errors, andso an inference concerning the global stress state in the blade root,and thus the torque acting there, is always affected by errors.

It is therefore the object of the present invention to provide a methodand a device that enable an improved determination of the load of arotor blade of a wind turbine system.

This object is achieved by the subject matter of the independent patentclaims. Advantageous refinements follow from the subject matters of thesubclaims, and from the following description.

The present invention provides a method for determining a bending angleof a rotor blade of a wind turbine system, the method having thefollowing steps:

-   -   reading in at least one acceleration signal that represents an        acceleration acting on the rotor blade, and    -   determining the bending angle of the rotor blade of the wind        turbine system by using the acceleration signal.

Furthermore, the present invention produces a device for determining abending angle of a rotor blade of a wind turbine system, the devicehaving the following features:

-   -   an interface for reading in an acceleration signal that        represents an acceleration acting on the rotor blade, and    -   a unit for determining the bending angle of the rotor blade of        the wind turbine system by using the acceleration signal.

Also advantageous is a computer program product having program code thatis stored on a machine readable carrier such as a semiconductor memory,a hard disk memory or an optical memory, and that is used to carry outthe method according to one of the abovedescribed embodiments when theprogram is executed on a control unit or a device.

The invention is based on the finding that the bending of the rotorblade of the wind turbine system is related in a predetermined fashionto a bending torque of this rotor blade at the blade root. In order todetermine the bending, use is made, in particular, of an acceleration oran acceleration signal that is measured in a fashion substantiallyperpendicular to the rotor plane. The acceleration in a longitudinaldirection of the blade can also be used as additional accelerationsignal. Denoted in this case as rotor plane is a virtual or actual planein which the rotor blades rotate about the rotor axis of the windturbine system. This means that the acceleration used in the presentapproach represents an acceleration in the direction of the rotor axis.Given knowledge of this predetermined relationship, it is possible inthis case to use the acceleration of the rotor blade, or at least of apart of the rotor blade, to infer the bending torque present at theblade root of this rotor blade so that a conventional control unit canfurther be used to determine the blade pitch angle by using a modifiedcontrol parameter. It is not mandatory in this case to infer the bendingtorque present at the blade root; rather, it is also possible todirectly calculate the incidence angle to be set on the basis of themeasured acceleration or acceleration read in. In this case, theacceleration determined is thus used to determine the blade deflection(that is to say a value beta), the IPC pitch angle or the incidenceangle of the rotor blades being determined therefrom. The preset controlcan therefore use the bending torque (or the bending angle), and thendetermines the so-called pitch angle of the rotor blade. It is alsopossible in this case to use the term “incidence angle” for the term“pitch angle”. Thus, the incidence angle or the individual incidenceangle for the rotor blade can be determined from the bending angle.

The present invention offers the advantage that conventional controlunits can continue to be used such that there is no need for a costintensive redevelopment of a control unit for controlling the incidenceangles of the rotor blades of the wind turbine system. At the same time,the sensor variables used can be provided by using sensors that aresubstantially more robust against aging phenomena and measuring errors.Because wind turbine systems are designed for a long running time and,in particular, it is very cost intensive to exchange rotor blades, theabovementioned advantage is gaining even further in importance. A simpleand cost effective retrofitting is also possible with the approachpresented here.

In accordance with a favorable embodiment of the present invention, aprofile of the acceleration can be acquired in the read-in step, in thedetermination step a spectrum being determined from the accelerationprofile, and the bending angle being determined by using the determinedspectrum. Such an embodiment of the present invention offers theadvantage that relatively small measuring errors can be compensated byusing a spectrum that has been determined over a specific period and wassubsequently transformed into the frequency domain. It is possible inthis case to utilize the fact that the revolution of the rotor blade orthe rotor blades periodically gives rise to physical influences atspecific positions in the pitch circle of the rotor blade, specificallyexactly when the rotor blade again reaches the specific position in thefollowing revolution.

It is particularly advantageous when in the determination step thedetermined spectrum is compared with a provided spectrum, the bendingangle being determined by using a result of comparison between thedetermined spectrum and the provided spectrum. Such an embodiment of thepresent invention offers the advantage of the possibility of a good andreliable determination of the provided spectrum. By way of example, itis possible in this case to determine a mean value from a multiplicityof recorded spectra, it then also being possible for such a providedspectrum to map certain variations of the environmental conditions.

In accordance with a particular embodiment of the present invention, inthe read-in step a movement of the rotor blade in a fashionsubstantially perpendicular to the rotor blade can be actively effected.Such an embodiment of the present invention offers the advantage thatspectra already to be expected for specific, frequently occurringscenarios can be measured or calculated and stored in a memory. Forexample, in this case a specific bending angle can be assigned to eachspectrum stored in the memory. In practical use, it is possible in thisway to determine the bending angle very easily numerically or bycircuitry, since in essence a comparison of the determined spectrum withone or more spectra from the memory needs to be performed in order toobtain an already very accurate magnitude for the bending angle from theresult of comparison when the determined spectrum correspondsapproximately to a specific spectrum to which this bending angle isassigned.

In accordance with a further embodiment of the present invention, in thedetermination step the acceleration signal can be subjected to lowpassfiltering and/or Kalman filtering. Such an embodiment of the presentinvention offers the advantage that filtering provides a smoothing ofthe measured value read in, which increases the stability of the controlmethod for the incidence angle. In particular, high-frequency signalinterference components are filtered out in this case, leaving the pureuseful signal which carries the desired information, to be evaluated,relating to the gravitation and centrifugal force.

Furthermore, in a further embodiment of the invention it is alsopossible in the determination step for determining the bending angle tomake use of an information item relating to bending stiffness or to anapproximation of the bending stiffness, an information item relating toa distance of an acceleration sensor providing the acceleration signalfrom a rotor axis, an inclination angle of the rotor axis from thehorizontal, and/or an acceleration of a tower head of the wind turbinesystem. Such an embodiment of the present invention offers the advantagethat a very precise estimate of the bending torque occurring at theblade root is possible hereby so that a slight change in theparametrization of control units already in use is necessary. This isrelevant, in particular, because the control units currently in usedetermine the control of the incidence angle for a rotor blade on thebasis of an occurring bending torque, and so the control variable can bevery easily exchanged.

In order to obtain a determination of the bending angle of the rotorblade that is as precise as possible, in the determination step it ispossible to determine a time profile of the acceleration at a positionof the rotor blade from the acceleration signal, and to determine thebending angle or the blade deflection of the rotor blade by using thedetermined profile. The time profile can in this case extend over arotor blade revolution about the rotor axis. Such an embodiment of thepresent invention determines the bending of the rotor blade by virtue ofthe fact that the position relative to the gravitational acceleration,which acts periodically during the measurement of the acceleration ofthe rotor blade in a fashion now amplifying and now reducing themeasured sensor signal, is determined.

In accordance with another embodiment of the present invention, in theread-in step it is possible to read in a further acceleration signalthat is measured in the direction of the longitudinal axis of the rotorblade. In this case, in the determination step of the bending angle ofthe rotor blade of the wind turbine system is determined by using thefurther acceleration signal.

The invention is explained in more detail in exemplary fashion belowwith the aid of the attached drawings, in which:

FIG. 1 shows an illustration of a uniform definition of designations ofthe possible movements on a wind energy system;

FIG. 2 shows a block diagram of a control unit for the individualincidence angles of a rotor blade of a wind energy system, in the caseof which an exemplary embodiment of the present invention can be used;

FIG. 3 shows a diagram of the principle of the relevant variables for apositioning of the sensor on a rotor blade;

FIG. 4 shows a diagram illustrating the relationship between a bladedeflection and a root bending torque, plotted against time;

FIG. 5 shows an illustration of a sensor coordinate system on a rotorblade;

FIG. 6 shows an illustration showing the measurement principle and theprocessing of the sensor signal obtained;

FIG. 7 shows a diagram in which a sensor signal subjected to lowpassfiltering and a reference signal are illustrated;

FIG. 8 shows a diagram illustrating the relationship between a bladedeflection and a useful signal of the acceleration measurement of asensor that is positioned at a distance of r=10 m from the rotor hub;and

FIG. 9 shows a flowchart in accordance with an exemplary embodiment ofthe present invention as method.

Identical or similarly acting elements can be provided in the followingfigures by identical or similar reference symbols. Furthermore, thefigures of the drawings, their description and the claims includenumerous features in combination. It is clear here to a person skilledin the art that these features can also be considered individually, orcan be brought together to form further combinations that are notexplicitly described here. Furthermore, in the following description theinvention may be explained by using different measurements anddimensions, although the invention is not to be understood as confinedto these measurements and dimensions. Furthermore, inventive methodsteps can be executed repeatedly and in a sequence other than thatdescribed. If an exemplary embodiment includes an “and/or” conjunctionbetween a first feature/step and a second feature/step, this can be readto the effect that, in accordance with one embodiment, the exemplaryembodiment has both the first feature/the first step and the secondfeature/the second step and, in accordance with a further embodiment,the exemplary embodiment has either only the first feature/the firststep or only the second feature/the second step.

A particular aim of the invention is to provide a possibility of using acontrol method to minimize the yaw and pitch torques on the nacelle,which result from asymmetric aerodynamic loads. Manipulated variablesare advantageously the individual incidence angles of the blades of thewind turbine system. An important aspect in this case is that, inaccordance with the approach presented here, the controlled variablesare determined via acceleration sensors on the rotor blades. To thisend, there is installed in at least one rotor blade at least oneacceleration sensor that can measure accelerations in the flappingdirection (that is to say perpendicular to the rotor plane). This offersthe advantage that it is possible hereby to use point sensors that caneasily be applied in the blades, are easy to exchange, and do notacquire static errors such as stresses owing to temperature differencesand the inhomogeneous blade material. In addition, in some circumstancesthe sensors are already present if condition monitoring of the blades isinstalled.

Recourse may be made to the illustration in accordance with FIG. 1 inthe interest of standard definition of the following variables usedregarding the possible movements of a wind turbine system. Here, a windturbine system is understood as a system having a tower on which anacelle is fastened. This nacelle contains a generator that is coupledto a rotor, the rotor having two rotor blades in the example illustratedin FIG. 1. In this case, the tower can execute a tower longitudinalbending 100 and a tower transverse bending 110 given an incident flow ofwind and a transmission of forces of the rotor onto the nacelle and thetower. Further, the tower can execute a tower torsion 120 about itsvertical axis. A movement of the tower about its vertical axis is alsodenoted as yawing 130 of the wind turbine system. Furthermore, it isalso possible for forces to act on the tower or the wind turbine system,which leads to a rolling 140, that is to say a rolling movement aboutthe rotor axis of the wind turbine system. If the effect of wind on thewind turbine system is to induce a movement that acts both perpendicularto the vertical axis of the tower and also to the rotor axis, the windturbine system is said to pitch 150. The rotor blades can, on the onehand, execute a pivoting movement 160 or a flapping movement 170, ortwist internally, this equally being denoted as torsion 180, referrednow to the rotor blades. The pivoting movement 160 corresponds in thiscase to a desired movement of the rotor blades about the rotor axis, theflapping movement 170 denoting a movement, in particular of the tips ofthe rotor blades, out of the rotor plane, that is to say in thedirection of extent of the rotor axis. Such a definition of movements ofa wind turbine system follows the definition from the book of E. Hau,entitled “Wind turbine systems”, in which corresponding controlledvariables are named for the yaw and pitch torque of the nacelle. Theflapping movement leads to bending torques on the blade root, and is thecause of yaw and pitch torques of the nacelle.

An important aspect of the present invention can be seen in that it ispossible to make use of acceleration signals from sensors on the blade,and to process these signals within a control method in order to reducethe yaw and pitch torques on the nacelle via the individual adjustmentof the blade incidence angles.

A system in the case of which the present invention can be used inaccordance with one exemplary embodiment is illustrated in a simplifiedfashion as a block diagram in FIG. 2. The system 200 for controlling thewind turbine system 210 comprises in this case a unit 220 foroperational control, and a unit 230 for controlling the individualincidence angle 235 (β_(IPC1,2,3)) for each of the rotor blades of thewind turbine system 210. From a sensor of the wind turbine system 210,the unit 220 for operational control (also denoted as CPC;CPC=Collective Pitch Control) receives a signal for outputting, inparticular a signal relating to the rotational speed ω of the rotor ofthe wind turbine system 210. The signal can now be used, on the onehand, by the unit 220 for operational control to determine a generatortorque 240 that is to be set, and to make this available for controllingthe wind turbine system 210 and, on the other hand, to determine for allrotor blades a common incidence angle 242 (β_(CPC)) for which the windturbine system has an optimum output efficiency. From at least onesensor in or on a rotor blade of the wind turbine system 210, thecontrol unit 230 (also denoted as IPC controller) for the individualincidence angle 235 receives a signal relating to an acceleration a₁ ofthis rotor blade at that position at which the sensor is arranged. Inparticular, the control unit 230 for the individual incidence angle 235can receive signals relating to accelerations a_(1,2,3) from a pluralityof, for example from all rotor blades, and in this case it can providefor each rotor blade for which it receives a sensor signal acorresponding signal β_(IPC1,2,3) for setting the individual incidenceangle 235 of the relevant rotor blade. In this way, the signal relevantto the common incidence angle can be corrected for each individual rotorblade in order to take account of local wind inhomogeneities.Furthermore, the shear of the wind also leads to asymmetric loads. Thesignal relating to the common incidence angle 242 can then, for example,be combined additively with the different signals relating to theindividual incidence angles 235 for the relevant rotor blades, thusyielding a control signal 250 for the individual relevant rotor bladesof the wind turbine system 210. This adjustment of the incidence angleof the individual rotor blades of the wind turbine system 210 inaccordance with the desired incidence angles is subsequently set by anactuator 255. Under the influence of varying wind conditions 260, therotor blades are then deflected in a flapping direction with differentdegrees of intensity, this deflection or the acceleration occurring inthis case being measured in turn by the appropriate sensors, and beingfed via the sensor signals 265 to the operational control unit 220 andto the control unit 230 for the individual incidence angle. In this way,the control loop for controlling the individual incidence angles isclosed.

A very simple modification of already existing control systems for theindividual incidence angles of the rotor blades of a wind turbine systemcan be implemented by using acceleration signals that represent anacceleration of the individual rotor blades in the flapping direction.Specifically, conventional wind turbine systems mostly use the bendingtorques at the blade root of the rotor blades to set the individualincidence angles of the relevant rotor blades. However, since a simplerelationship between a bending torque at the blade root of a rotor bladeand an associated bending of the rotor blades in a flapping directioncan mostly be detected, or is known, it is possible by means of a signalof a substantially more robust acceleration sensor to use for thecontrol of the incidence angle of the rotor blade an adequately usefulsignal that represents the acceleration of the rotor blade or of a partof the rotor blade in a flapping direction by employing this signal todetermine the bending angle of the rotor blade in accordance with theinvention. Two variants can now be conceived in order to obtain and toprocess a signal relating to a bending angle of a rotor blade that canbe processed well and is as free from interference as possible.

In a first variant, a natural frequency analysis of the determinedaccelerations or of the acceleration signals derived therefrom can becarried out. To this end, use is made of the natural vibrations of the(rotor) blade. The excitation during the operation of the system isperformed by aerodynamically induced vibrations or via an additionallyfitted shaker, that is to say a unit which actively sets the rotor bladevibrating. In this case, the acceleration sensors continuously acquireand store signals, and determine the amplitude spectrum of the naturalvibrations after a specific measuring time (at most 1 s). This frequencyspectrum is, for example, compared with desired spectra that are storedin the control/regulation device and are associated with specificloading states of the blade. Loading at the blade is reduced byadjusting the blade angle, and this is controlled by comparison with thedesired spectra. The desired spectra are preliminarily determined bymeasurements on the blade without and with loadings, or else determinedfrom calculations via natural frequency analysis. The advantage of thisvariant consists in that it is also possible to make use of the alreadyavailable measurement technique for condition monitoring, which hasalready integrated the sensors, the acquisition of measured values, andthe preparation and evaluation of the acceleration signals. Desiredspectra already stored are also associated therewith. Desired spectrafor loading cases that are stored in the control or regulation deviceshould be added thereto for such an application scenario. Said spectracan be determined by measurements on blade test stands. It is probablyeasier to carry out reference measurements before mounting on the blade,and to carry out similar measurements on the rotor blade after mountingin the case of wind speeds below the startup speed. The deviationsrelating to these spectra in the case of loading are calculated viasimulation starting from these spectra and the blade data, and stored asdesired spectra.

A second variant for the use of the approach presented here is to beseen in the use of data from a direct acceleration measurement andevaluation thereof. In this case, the bending angle of the rotor bladeis determined from the measured accelerations. The control aim is thento set the same bending angles at all rotor blades. The blade angles areonce again manipulated variables. Because of aerodynamic effects such asturbulence and vortex shedding, vibrations of the blade are alwaysexcited, but they are of higher frequency than the vibrations to beremoved by control in the region of the first natural frequency of bladeand tower. Consequently, for control purposes the measured accelerationshould be filtered by means of a lowpass filter. The lower half of therotor blade is advantageous for the position of the first (acceleration)sensor, since the blade tip can be excited to vibrate strongly owing tothe tapering and the transverse flows prevailing there, which also drivethe tip vortex.

The following aspects may be adduced as advantages of the twoabovedescribed variants. Firstly, it is possible to use known andpossibly already present measuring devices and, if appropriate, datadetermined by the condition monitoring of the blades. Furthermore, thereis no need for any application of strain gauges or the like, in the caseof which it is not known in the prior art where and how they are to beexactly fitted. In addition, the temperature compensation for thesesensors has not yet been satisfactorily resolved in technical terms. Inaddition, an acceleration sensor can easily be replaced in the case of adefect. This is impossible with strain sensors that have been laminatedin. The signals supplied by strain sensors are possibly not indicative,since they acquire only the local strain. Again, when the abovedescribedapproach is used there is no occurrence of errors owing to staticloadings such as temperature stresses, excessive local stresses owing tothe inhomogeneous material, ice coating (with simultaneous use ofcondition monitoring) etc., something which greatly increases thereliability of the control by making use of the variable of the bendingangle which is calculated from the blade acceleration.

In other words, this means that the approach presented here constitutesa use as additional control function with the pitch drives of theapplicant. On the basis of current market trends, future drives shouldbe able to adjust the blades individually.

A further important aspect of the present invention consists inenabling, on the basis of an acceleration sensor (DCU), an improvedIPC-suited measurement method in which the sensor has a longer servicelife, a simple exchangeability of the sensor system is ensured, and avariable equivalent to the global stress state in the blade root isacquired as far as possible.

In the case of the approach presented below, a substantial aspectresides in the use of a signal of an acceleration sensor that measuresthe acceleration of the rotor blade in the direction of the rotor axis.The acceleration sensor should be able to measure stationaryacceleration. The measurement of the acceleration in the blade is knownaccording to the prior art, and is used, inter alia, for conditionmonitoring. A twofold integration of this measured acceleration wouldprovide the current blade deflection. However, this method has a driftthat corrupts the calculated results over a longer time. This measuredvariable is therefore not suitable for IPC control.

In accordance with an exemplary embodiment, the invention presented herepresents a measurement concept that enables a signal evaluation suitablefor the IPC control. An online signal evaluation can be performed on thebasis of sensor data of a mono-axial acceleration sensor. A possible useresides, for example, in the field of IPC control, or with experimentalmeasurements on wind energy systems. The control of the blade angles ofthe rotor blades of a wind energy system requires the blade deflectionin a flapping direction (that is to say perpendicular to the rotor planegiven 0 degree pitch setting). In order to determine this variable, theblade deflection can be measured directly via strain gauge sensors onthe rotor blade root. An alternative sensor concept for the measurementof the blade deflection is the use of acceleration sensors whosemeasurement equation is described by the so-called navigation equation(3), which reads as follows:

$\begin{matrix}{{\overset{\rightarrow}{a} = {\left. \frac{^{2}\overset{\rightarrow}{r}}{t^{2}} \middle| {}_{i}{+ \overset{\rightarrow}{g}} \right. = \left. \frac{{\overset{\rightarrow}{v}}_{i}}{t} \middle| {}_{i}{+ \overset{\rightarrow}{g}} \right.}},} & (3)\end{matrix}$

a corresponding to the measured acceleration, and g to the gravitationalacceleration.

Given appropriate lowpass filtering of the local acceleration, thesensor signal can be used to estimate the projection of thegravitational vector, and thus the pitch angle of the sensor coordinatesystem. The orientation of the sensor can be used to infer thedeflection of the rotor blade, and thus the corresponding flappingbending torque. The diagram from FIG. 4 shows a measured relationshipfor the deflection of the rotor blade and the corresponding flappingbending torque, in which time is represented on the abscissa, and theprofile of the blade deflection (dashed line) and of the blade rootbending torque (continuous line) is represented on the ordinate. It isto be seen in this case from FIG. 4 that the profiles for the measuredblade deflection and the measured blade root bending torque correspondto one another such that in order to control the individual incidenceangle of the rotor blade it is also possible to use the bladedeflection, and thus also the acceleration that leads to the relevantblade deflection.

Assuming negligible torsion, it suffices to take account of thex-component of the sensor signal in order to determine the bladedeflection. The x-component points in the direction of the normal vectoron the blade surface, and lies in the bending flapping direction to theextent that no blade torsion is present. A sensor coordinate system 500in the rotor blade, such as is shown in FIG. 5, is assumed to this end.In this case, the z-component is oriented in the direction of the rotorblade end, the x-component in a normal to the rotor plane, and they-component in a pivoting direction of the rotor blade. Furthermore, acoordinate system 510 in the hub of the rotor, and a coordinate system520 in the rotor shaft can be used for the conversion of the sensoracceleration values, as is further described in more detail below.

In order to determine the blade deflection, the first step to this endis to transform the coordinates of the tower into the rotor axis, thecoordinates of the rotor axis into the rotor blade, and from the rotorblade into the bent rotor blade. The following transformation matricescan be used to this end:

${M_{{tower}\mspace{14mu} {into}\mspace{14mu} {rotor}\mspace{14mu} {axis}} = {\begin{pmatrix}{\cos \; \Lambda} & 0 & {{- \sin}\; \Lambda} \\0 & 1 & 0 \\{\sin \; \Lambda} & 0 & {\cos \; \Lambda}\end{pmatrix} = M_{ST}}},{M_{{rotor}\mspace{14mu} {axis}\mspace{14mu} {into}\mspace{14mu} {rotor}\mspace{14mu} {blade}} = {\begin{pmatrix}1 & 0 & 0 \\0 & {\cos \; \Omega} & {\sin \; \Omega} \\0 & {\sin \; \Omega} & {\cos \; \Omega}\end{pmatrix} = M_{\; {BS}}}},{and}$${M_{{rotor}\mspace{14mu} {blade}\mspace{14mu} {into}\mspace{14mu} {deflected}\mspace{14mu} {rotor}\mspace{11mu} {blade}} = {\begin{pmatrix}{\cos \; \beta} & 0 & {{- \sin}\; \sin \; \Omega} \\0 & 1 & 0 \\{\sin \; \beta} & 0 & {\cos \; \beta}\end{pmatrix} = M_{B^{\prime}B}}},$

Λ representing the inclination angle of the rotor axis relative to ahorizontal, Ω representing the rotor azimuth angle about the rotor axis,and β representing the torsion angle of the rotor blade at the locationof the sensor from the rotor plane. In this case, a projection of thegravitational acceleration

${\underset{\_}{g}}_{T} = \begin{pmatrix}0 \\0 \\{- g}\end{pmatrix}_{T}$

can be described in the following way:

${\underset{\_}{g}}_{T} = {{M_{B^{\prime}B} \cdot M_{BS} \cdot M_{ST} \cdot {\underset{\_}{g}}_{B^{\prime}}} = {g \cdot {\begin{pmatrix}{{\cos \; {\beta \cdot \sin}\; \Lambda} + {\sin \; {\beta \cdot \cos}\; {\Omega \cdot \cos}\; \Lambda}} \\{{- \sin}\; {\Omega \cdot \cos}\; \Lambda} \\{{\sin \; {\beta \cdot \sin}\; \Lambda} - {\cos \; {\beta \cdot \cos}\; {\Omega \cdot \cos}\; \Lambda}}\end{pmatrix}.}}}$

Furthermore, a measurement equation of the acceleration sensors can bespecified as follows:

${\underset{\_}{a} = {{\left( \frac{}{t} \right)^{t}\left\lbrack {\left( \frac{\;}{t} \right)^{f}\underset{\_}{r}} \right\rbrack} + \underset{\_}{g}}},$

from which it follows that:

$a_{{Sensor}^{\prime}} = \begin{pmatrix}{{\omega^{2} \cdot \cos}\; {\beta \cdot \sin}\; {\beta \cdot r_{s}}} & {{+ r_{s}} \cdot \overset{\_}{\beta}} & {{+ g} \cdot \left( {{\cos \; {\beta \cdot \sin}\; \Lambda} + {\sin \; {\beta \cdot \cos}\; {\Omega \cdot \cos}\; \Lambda}} \right)} \\0 & {r_{s} \cdot \left( {{2{\omega \cdot \sin}\; {\beta \cdot \beta}} - {{\overset{.}{\omega} \cdot \cos}\; \beta}} \right)} & {{- g} \cdot \left( {\sin \; {\Omega \cdot \cos}\; \Lambda} \right)} \\{{- \omega^{2}} \cdot \left( {\cos \; \beta} \right)^{2} \cdot r_{s}} & {{- r_{s}} \cdot {\overset{\_}{\beta}}^{2}} & {{+ g} \cdot \left( {{\sin \; {\beta \cdot \sin}\; \Lambda} - {\cos \; {\beta \cdot \cos}\; {\Omega \cdot \cos}\; \Lambda}} \right)}\end{pmatrix}_{B^{\prime}}$

the first column of the above-specified matrix representing thecentripetal acceleration, the second column of the above-specifiedformula the measured accelerations on the basis of the rotation of thesensor coordinate system, and the third column of the above specifiedformula the gravitational acceleration.

If the tower head acceleration is not to be neglected, it is necessaryto expand the sensor equation by a_(tower head), where

$a_{{tower}\mspace{14mu} {head}} = {\begin{pmatrix}{{+ {a_{x}\left( {{\cos \; {\beta \cdot \cos}\; \Lambda} - {\sin \; {\beta \cdot \sin}\; {\Lambda \cdot \sin}\; \Omega}} \right)}} + {{a_{y} \cdot \sin}\; {\beta \cdot \sin}\; \Omega}} \\{{{{+ a_{x}} \cdot \sin}\; {\Lambda \cdot \sin}\; \Omega} + {{a_{y} \cdot \cos}\; \Omega}} \\{{+ {a_{x}\left( {{\sin \; {\beta \cdot \cos}\; \Lambda} + {\cos \; {\beta \cdot \sin}\; {\Lambda \cdot \cos}\; \Omega}} \right)}} - {{a_{y} \cdot \cos}\; {\beta \cdot \sin}\; \Omega}}\end{pmatrix}.}$

Consequently, it therefore holds for the measured total accelerationa_(sensor) of the sensor that:

a _(sensor) =a _(sensor′) +a _(tower head).

The components based on the rotation of the sensor coordinate system canbe filtered by a lowpass filter and thereby eliminated.

In accordance with the exemplary embodiment of the present inventionpresented here, it is possible to implement two measurement concepts ormeasurement methods. A mono-axially measuring acceleration sensor on therotor blade is used for the first method. What is measured in this caseis the acceleration that is, for example, directed normally onto therotor blade surface. This acceleration is denoted as a_(x, sensor) andcan be expressed as follows, neglecting the tower head acceleration:

a _(x,Sensor)=ω²·cos β·sin β·r _(s) +r _(s) ·{umlaut over (β)}+g·(cosβ·sin Λ+sin β·cos Ω·cos Λ),

the term g·(cos β·sin Λ+sin β·cos Ω·cos Λ) periodically repeated withthe angle Ω. Furthermore, when β is small, and thus sin β tends to 0,the trajectory of the gravitational acceleration can be used uniquely todetermine β.

Without blade deflection, it therefore holds that β=0. It followsherefrom that

(a _(x))_(sensor,filtered) =+g·sin Λ=const.

The result with β≠0 is a rotational frequency component (sin β·cos Ω·cosΛ), it being possible to determine the bending angle from the followingrelationship:

${2 \cdot g \cdot \left( {\sin \; {\beta \cdot \cos}\; \Lambda} \right)} = {\left. A\Rightarrow\beta \right. = {{arc}\; {{\sin \left( \frac{A}{{2 \cdot g \cdot \cos}\; \Lambda} \right)}.}}}$

The change in the projection trajectory of the gravitationalacceleration is therefore determined by the deflection of the rotorblade (β≠0), and can be used to determine β. The first measurementmethod of the blade deflection on the basis of a mono-axially measuringacceleration sensor offers advantages with reference to a sensor thatcan be used cost effectively, and to a simpler evaluation of the sensorsignals than in the case of the use of a plurality of sensor signals.However, it must be adduced as a disadvantage of this measurement methodfor the bending angle that a smaller useful signal is available, becauseonly the signal amplitude can be used to determine β.

Variables as explained in more detail with reference to FIG. 3 can beused for the second method for determining the bending angle. Here, theacceleration of the rotor blade 300 in the direction of the rotor axis310 is considered, the acceleration sensor being arranged at thedistance r therefrom. The following accelerations are measured by thelocal rotation of the rotor blade at the location of the accelerationsensor by the angle β owing to the deflection of the rotor blade fromthe rotor plane (320):

a _(x,Sensor)=ω²·cos β·sin β·r _(s) +r _(s) ·{umlaut over (β)}+g·(cosβ·sin Λ+sin β·cos Ω·cos Λ)+a _(x)(cos β·cos Λ−sin β·sin Λ·sin Ω)+a_(y)·sin β·sin Ω

and

a _(x,Sensor)=−ω²·(cos β)² ·r _(s) −r _(s)·{dot over (β)}² +g·(sin β·sinΛ−cos β·cos Ω·cos Λ)+a _(x)(sin β·cos Λ+cos β·sin Λ·cos Ω)−a _(y)·cosβ·sin Ω.

The first term (that is to say the first product) of the two equationsis constant in this case with reference to the angle β. The second term(that is to say the second product) is negligible when the accelerationsensor signal is subjected to lowpass filtering. The last term (that isto say the last product) is periodic with the angle Ω.

By way of example, a constant component of ω²·r_(s)=58 m/s² can beobtained given an angular velocity of ω=1.7 rad/s (which corresponds toa system rotational speed of 15 rpm) and a distance of the sensorr_(s)=20 m. The rotational frequency component is g=9.81 m/s² in thiscase. Consequently, the acceleration in the z-direction fluctuates, byway of example, from 68 m/s² to 48 m/s² within a revolution of the rotorblade above the rotor axis. All variables except β are now known in thefiltered equation for a_(z,sensor) (that is to say the second term isfiltered out). The equation can therefore be solved numerically for thedesired torsion angle β.

$\left. {{{\omega^{2} \cdot r_{s} \cdot \cos^{2}}\beta}}\Rightarrow\beta \right. = {{arc}\; \cos \sqrt{\frac{a_{z}}{\omega^{2} \cdot r_{s}}}}$

The above-specified equation for a_(x,sensor) specifies how theacceleration measured in the x-direction is composed of the known andunknown variables. If this acceleration is additionally measured, theaccuracy of the determination of β can be increased. In particular, theuse of a Kalman filter can lead to better results. In this case, a modelof the rotor blade is simulated in the Kalman filter and the deflectionis determined therefrom. The simulation is updated and/or corrected (forexample, by means of a predictor, corrector method) in each time stepwith the aid of the two measurements (a_(x), a_(z)). The bladedeflection can be determined directly from the blade inclination withthe aid of a model for the blade bending (that is to say the bendingline). The blade root bending torque then also follows from the modelfor the blade bending. The bending stiffness EI so also requires to beknown for this. Since the known IPC controllers use only the differencesin the blade root bending torques of the blades for control purposes,there is no need for an absolutely accurate value, and an approximatevalue suffices for the bending stiffness EI. Such a previously mentionedmeasurement concept could also be established simply by the presence ofan acceleration sensor in the rotor blade which is arranged formeasuring the acceleration in a z-direction.

This the application of the above-described equations, it is thenpossible to use the acceleration signals to infer the bending angle ofthe rotor blade, which is then further used to control the incidenceangle of the rotor blade. In particular, in this case the application ofthe second method has the advantage that because of a constantcentrifugal force there is present over a rotor revolution a constantuseful signal from which the g-projection can then be calculated or alsoused to determine β.

The projection component of the gravitational acceleration, which is tobe ascribed purely to the flapping bending of the rotor blade, should bedetermined in order to be able to determine the blade deflection. Tothis end, it is possible to filter out the change in projection of thegravitational acceleration, which results from the rigid body movementof the system. Two degrees of freedom determine the rigid body movement:the rotation about the rotor axis and the blade angle adjustment aboutthe pitch axis. In addition, the rotation of the azimuth bearing couldalso be considered, but this is neglected in this consideration.

The projection component responsible for the blade deflection istherefore yielded from:

{right arrow over (g)} _(Bending) ={right arrow over (g)} _(Mess)−{right arrow over (g)} _(RBFilter)   (4)

g_(RBFilter) corresponding to the projection vector of the gravitationalacceleration, which is calculated on the basis of the rigid bodymovement. g_(Mess) is the gravitational acceleration component measuredby the sensor. g_(Bending) is the appropriately filtered signal, whichis to be ascribed purely to the elastic deformation of the rotor blade(that is to say corresponds to the bending in a flapping direction).Equation (4) can be used to filter out the projection component of thegravitational vector, which is not to be ascribed to the deflection. Thecalculation of the projection of the gravitational acceleration, whichresults from the rigid body movement, may be gathered from the followingequation (5).

{right arrow over (g)} _(RBFilter) =T _(Blade) _(—) _(Hub) ·T _(z)(β)·T_(Hub) _(—) _(Rotor) ·T _(x)(α)·{right arrow over (g)} _(RotorCOS)   (5)

where

-   -   T_(Blade) _(—) _(Hub) corresponds to a transformation matrix for        a transformation into the blade segment COS,    -   T_(z)(β) represents a rotation by β (that is to say a pitch        angle of the rotor blade) referred to the z-axis of the blade        bearing COS,    -   T_(Hub) _(—) _(Rotor) corresponds to a transformation matrix for        a transformation into the blade bearing COS, and    -   T_(x)(α) corresponds to a rotation by α (that is to say an        azimuth angle of the rotor) referred to the x-axis of the rotor        COS.

In this case, {right arrow over (g)}_(RotorCOS) denotes thegravitational vector expressed in the inertial rotor axis coordinatesystem. The rotor axis is upwardly inclined by approximately 5° by theso-called shaft angle.

The measurement principle in accordance with the first method isillustrated in the two partial figures of FIG. 6. In this case, theleft-hand partial figure illustrates a measurement principle and anassociated measurement signal, in the case of which the wind turbinesystem and/or the rotor blades are deflected. By contrast, theright-hand partial figure from FIG. 7 illustrates a measurementprinciple and an associated measurement signal in the case ofapplication of this measurement principle, it being possible to inferthe elastic deformation of the rotor blade from the variation in theamplitude.

The diagrams of FIGS. 7 and 8 represent the sensor signal (dashed line700) at the blade tip (that is to say at a distance of r=36 m from theblade root) and the sensor signal subjected to lowpass filtering(continuous line 710) plotted against time, whereas FIG. 8 representsthe profile of a sensor signal subjected to lowpass filtering. Thereference signal corresponds in each case to the projection of thegravitational acceleration into the sensor coordinate system. It is tobe seen that the lowpass filtering enables determination of theamplitude of the projection of the gravitational vector. The amplitudeof the projection is relevant for the evaluation of the bladedeflection.

Furthermore, there is a correlation between the amplitude of the bladedeflection and the filtered sensor signal. Here, it is necessary to takeaccount of the change in the g-projection, which may be ascribed to thedeflection of the rotor blade. This means that a relatively largeamplitude signal is also to be expected given a relatively largedistance of the sensor from the blade root. This information can also beobtained from the amplitudes of the variable {right arrow over(g)}_(Bending). Consequently, the blade root (flapping) bending torquescan be determined directly with the aid of the blade accelerationsensors in the course of an appropriate calibration. It may be shownthat a better evaluation is possible on the basis of the larger bladebending given a distance of r=20 meters of the acceleration sensorrelative to the rotor hub. There is an analogous result given a distanceof r=36 meters of the acceleration sensor relative to the rotor hub, thesimulation results not being illustrated here.

However, there is still a problem in that the useful signal, that is tosay the change in the inclination of the gravitational vector on thebasis of the blade bending, is relatively small in relation to theinterference signal the more closely the sensor is applied on the bladeroot. It is correspondingly more advantageous to measure further out onthe blade, for example at a position of r=36 meters from the blade hub,because of the larger deflection by comparison with the measurementpoints located relatively close to the blade bearing at, for example,r=10 and r=20 meters. Furthermore, the measurement of the rotorrotational speed and of the pitch angle are important for theapplication of the measurement concept presented here. However, this iscurrently prior art for wind energy systems, and is used forconventional control methods.

Furthermore, the approach presented here enables an already well maturedrange of acceleration sensors (for example MM3, DCU) of the applicant tobe used for the sensor signal evaluation described here, and saidapproach is capable of future use within a larger scope in the field ofthe control of wind energy systems.

In accordance with a further exemplary embodiment, the present inventioncomprises a method 900 for determining an incidence angle of a rotorblade of a wind turbine system as illustrated in the form of a flowchartin FIG. 9. The method 900 has a step of reading in 910 an accelerationsignal that represents an acceleration of the rotor blade actingsubstantially perpendicular to a rotor plane of the wind turbine system.Furthermore, the method 900 comprises a step of determining 920 theincidence angle of the rotor blade of the wind turbine system by usingthe acceleration signal.

LIST OF REFERENCES

100 Tower longitudinal bending

110 Tower transverse bending

120 Tower torsion

130 Yawing

140 Rolling

150 Pitching

160 Pivoting movement

170 Flapping movement

180 Torsion

200 System for controlling the wind turbine system

210 Wind turbine system

220 Operational control unit

230 Control unit for the individual incidence angle

235 Individual incidence angles (β_(IPC1,2,3))

240 Generator torque

242 Common incidence angle (β_(CPC))

250 Control signal

255 Actuator

260 Local wind conditions

265 Sensor signals

300 Rotor blade

310 Rotor axis

320 Rotor plane

500 Coordinate system in the rotor blade

510 Coordinate system in the rotor hub

520 Coordinate system in the rotor shaft

700 Reference signal

710 Filtered sensor signal

900 Method for determining a bending angle of a rotor blade

910 Reading in an acceleration signal

920 Determining the bending angle of the rotor blade of the wind turbinesystem

1. A method for determining a bending angle of a rotor blade of a windturbine system comprising: reading in at least one acceleration signalthat represents a first acceleration acting on the rotor blade in afashion substantially perpendicular to a rotor plane; and determiningthe bending angle of the rotor blade of the wind turbine system by usingthe at least one acceleration signal.
 2. The method as claimed in claim1, wherein: reading in the at least one acceleration signal includesacquiring an acceleration profile, determining the bending angle of therotor blade includes determining a spectrum from the accelerationprofile, and determining the bending angle includes using the determinedspectrum.
 3. The method as claimed in claim 2, wherein: determining thebending angle of the rotor blade includes comparing the determinedspectrum with a provided spectrum, and using a result of the comparison.4. The method as claimed in claim 2, wherein reading in the at least oneacceleration signal includes actively exciting vibration of the rotorblade.
 5. The method as claimed in claim 1, wherein determining thebending angle of the rotor blade includes subjecting the at least oneacceleration signal to at least one of lowpass filtering and Kalmanfiltering.
 6. The method as claimed in claim 1, wherein: determining thebending angle includes using an information item relating to at leastone of: a distance of a first acceleration sensor providing the at leastone acceleration signal from a rotor axis, an inclination angle of therotor axis from horizontal, and an acceleration of a tower head of thewind turbine system, and determining the bending angle includes using aninformation item relating to rotational speed and rotary position of arotor.
 7. The method as claimed in claim 1, wherein determining thebending angle includes considering a time profile of the firstacceleration at a position of the rotor blade.
 8. The method as claimedin claim 6, further comprising reading in a further acceleration signalthat represents a second acceleration, acting substantially in alongitudinal direction of the rotor blade, at a location of the firstacceleration sensor, wherein determining a bending angle of the rotorblade of the wind turbine system includes using the further accelerationsignal.
 9. The method as claimed in claim 1, further comprising one of:determining individual incidence angles of the rotor blades based on thebending angle, and determining individual incidence angles of the rotorblades based on loading, determined from the bending angle, of the rotorblade or the rotor blades.
 10. A device for determining a bending angleof a rotor blade of a wind turbine system, the device comprising: aninterface configured to read in at least one acceleration signal thatrepresents an acceleration acting on the rotor blade, and a unitconfigured to determine the bending angle of the rotor blade of the windturbine system by using the acceleration signal.
 11. A computer programproduct having program code for carrying out a method for determiningthe bending angle of a rotor blade of a wind turbine system, thecomputer program product comprising: a mechanism configured to read inat least one acceleration signal that represents a first accelerationacting on the rotor blade in a fashion substantially perpendicular to arotor plane; and a mechanism configured to determine the bending angleof the rotor blade of the wind turbine system by using the at least oneacceleration signal, wherein the computer program product is configuredto determine the bending angle when the program is executed on one of acontrol unit and a device.